The invention relates to a high pressure discharge lamp having a ceramic metal halide arc tube enclosing a discharge space with a pair of electrodes, and means for generating ultraviolet (UV) radiation which produces electrons in the discharge space.
U.S. Pat. No. 4,818,915 discloses a ceramic metal halide discharge lamp having a UV-enhancer in the form of a so-called glow bottle having an electrode in an envelope of UV transmitting borosilicate glass or quartz. The electrode is connected to a lead-in of one of the arc tube electrodes, and the envelope is capacitively coupled to the other arc tube lead-in. Application of an ignition voltage pulse across the arc tube electrodes creates an electric field passing through the fill gas of the glow bottle, which is preferably inert gas and mercury. This in turn produces UV radiation which stimulates emission of electrons from at least one of the electrodes in the discharge space by the photoelectric effect. These photoelectrons enhance gas breakdown and initiation of a discharge between the arc tube electrodes, thereby reducing starting time of the lamp. However ignition voltage pulses on the order of 5 kV are required to produce UV radiation.
U.S. Pat. No. 5,811,933 discloses a UV-enhancer in the form of a glow bottle having an envelope of ceramic material such as polycrystalline alumina (PCA) and a filling of inert gas. The use of ceramic reduces the minimum required ignition pulse voltage for reliable ignition to under 3 kV without appreciably increasing ignition time. The ceramic also has very good heat resistance, which renders it possible to position the UV enhancer very close to the arc tube. Capacitive coupling to the arc tube electrode is thereby achieved directly, without any additional conductor.
U.S. Pat. No. 5,541,480 discloses a high pressure discharge lamp with a ceramic metal halide arc tube having an outer surface on which a metallic coating is present. The coating extends along the length of the arc tube between electrodes and serves as a so-called ignition strip or starting antenna. The antenna capacitively couples the high voltage pulse from an electrode, through the fill gas and the ceramic, to the antenna, and finally to the other electrode. This reduces the apparent distance between electrodes and therefore increases the applied electric field which accelerates primary electrons and initiates the so-called Townsend avalanche. This occurs when at least one secondary electron is emitted in the fill gas for each primary electron, and the discharge current becomes self-sustaining.
U.S. Pat. No. 5,661,367 discloses a lamp having side-by-side ceramic metal halide arc tubes connected in series, with a metal strap capacitively coupling electrodes of respective arc tubes. The arc tubes each have a central barrel with opposed end plugs and electrodes extending therethrough, and the strap has end portions looped around electrically opposed end plugs close to the respective barrels. A gap is present between the electrodes and the inside surface of the end plugs. According to the specification, the electric field induces ionization of the fill gas in this gap. This produces radiation which in turn ensures emission of electrons from the electrodes. Once a gas discharge is supported, the impedance of the arc tube is reduced and the other arc tube sees the entire energy of subsequent starting pulses.